\subsubsection{Envelope Detection}
The plan for the demodulation was to use envelope detection. The circuit that was used for this is shown in \cref{fig:envelopecircuit}.

\begin{figure}
\centering
\includegraphics{assets/EnvelopeDetection.png}
\caption{Envelope detection circuit}
\label{fig:envelopecircuit}
\end{figure}

The transistor with the two diodes work like a diode, but instead of the positive part, it lets the negative part of the signal through and it also gives the signal an offset. The low pass filter at the end should give the data signal that was modulated onto the carrier signal.

However, because of the diodes in the circuit it has some nonlinear behavior. Because of this, the output of the circuit was not just the enveloped signal, but also a lot of disturbances at 13.56 MHz caused by this nonlinear behavior. 
The signal could have been filtered digitally and then used. Unfortunately there was no hardware available during the project that was fast enough to sample the signal.
In order for a lower sample frequency to be sufficient, the filter should be filtered. Another, and probably better, solution would be to use IQ detection. The reader (using the chip RC522) that was used during this project does also use a IQ detection. The advantage of using IQ detection is that you get the same result as when using a synchronous detection method but you do not need to establish a phase locked loop.